Patent GB-905462-A (GB)
Title: Electric control apparatus employing magnetic amplifiers
IPC: H02P9/32
Filed: 1960-06-22
Abstract: 905,462. Automatic voltage-control systems. WESTINGHOUSE ELECTRIC CORPORATION. June 22, 1960 [June 25, 1959], No. 21856/60. Class 38 (4). [Also in Group XXXIX] An automatic control system comprises a magnetic amplifier, having main control windings, the amplifier being responsive to the sum of first and second input signals and having an additional winding to which the difference of the input signals is applied so that the output of the magnetic amplifier is dependent on the larger of the input signals only. As shown, Fig. 1, a load 100 is supplied with direct current from an alternating current supply 102, 104 through a full-wave rectifier 92 and a doubler-connected magnetic amplifier 60 which is arranged to be responsive to the larger of two control signal currents I 1 , I 2  from devices 40, 50, respectively. The series circuit comprising the device 40 and control windings 46, 48 and series circuit comprising the device 50 and control windings 56, 58 are connected in parallel across terminals 101, 103 of a bridge rectifier 96. Control windings 72, 74 are connected to terminals 105, 107 of the bridge and carry a difference signal current I D  equal to the difference between the control signal currents I 1 , I 2  which has a fixed direction of flow due to the polarity of the rectifier 96. The turns comprising the control windings being equal, then for I 1  greater than I 2 . NI 1 +NI 2 +N(I 1  - I 2 ) = 2 NI 1  and the magnetic amplifier is controlled by the signal current from device 40 only, conversely, when I 2  is greater than I 1  then NI 1 +N(N 2  - N 1 ) and the magnetic amplifier is controlled by the signal current from device 50 only. In a system for controlling the voltage of a three-phase alternator 210, Fig. 3, supplying lines 214-218 a field winding 212 is energized from an exciter 220 having a boost field winding 226 and a back field winding 228 controlled by a push-pull magnetic amplifier 240. A voltage error signal current between terminals 347, 346 is fed to control windings 330-336 and a minimum excitation signal current is fed to control windings 314-320 and a difference current flows through the control windings 302-308 in a direction determined by the polarity of rectifier 362. The voltage error signal is normally larger than the minimum excitation signal current so that the exciter field windings are energized so as to maintain the alternator line voltage constant. When the line voltage falls excessively due to overload the minimum excitation signal current exceeds the voltage error signal and the alternator is controlled to limit the reduction in excitation. The error current signal is derived from a circuit 350 wherein a direct voltage representative of the line voltage derived through a rectifier 326 and transformer 323 is compared with the voltage of a reference source 315. The minimum excitation signal current is derived from a circuit 394 wherein a voltage developed across a resistor 392 by a current transformer 342 is added vectorially to a voltage across a secondary 384 of a transformer 380 connected across the secondaries of transformers 323. The resultant voltage is rectified and applied to a portion 368B of a resistor 368. A direct voltage across portion 368A, is opposing the voltage across portion 368B is proportional to line voltage and is derived through a rectifier 394 and transformer 370.